Peel the parsnips and cut each into 5 hunks. Put in a large pan, totally covered with salted water and bring to the boil. Put a lid on top, reduce the heat to medium and simmer for 20 minutes, then drain. They should be tender enough to slip off a small sharp knife.
Meanwhile finely chop the spring onions and parsley. Strip the thyme leaves from the stems and set all of these aside.
Return the parsnips to the pot with the butter, cream and seasoning. Vigorously mash and then use a hand-held beater to make them really fluffy. Finally, fold in the greens and season to serve.